I. Field of the Invention
The present invention relates to a method for manufacturing an information recording medium and, more particularly, to a method for forming a dielectric layer for an information recording medium of the type from which information is reproduced according to the stray capacitance.
II. Description of the Prior Art
Video discs in which video signals are recorded at a high density have recently been developed and are already in the process of actual application. As an example of a video disc of this type, there is known a video disc from which information is reproduced according to changes in the stray capacitance. A video disc of this type is manufactured by forming pits corresponding to information in the surface layer of a disc-shaped substrate of a resin, forming a conductive layer on the surface of the substrate including the pits, and forming a dielectric layer of an organic polymer on the conductive layer. In order to reproduce information recorded on this video disc, a playback stylus with a flat front surface and an electrode on its one side surface is brought into contact with the surface of the video disc such that the front surface of the stylus faces the surface of the video disc. The video disc and the playback stylus are moved relative to each other. Then, changes in the capacitance between the disc and the electrode are detected which depend upon the presence or absence of the pits on the surface of the disc.
In video discs of the types from which information recorded as an indented pattern is reproduced in accordance with changes in the stray capacitance, including the video disc as described above, the dielectric layer which is formed to produce the stray capacitance is required to be uniformly extremely thin and void of defects.
In order to form such a dielectric layer, there are conventionally known a method involving spin coating and curing a thermosetting or photocuring resin paint, a method involving placing a disc-shaped substrate in an atmosphere containing a monomer and polymerizing the monomer by glow discharge, a method involving forming a polymer on the substrate by sputtering, and the like.
However, the method using a resin paint has several drawbacks. For example, the dielectric layer tends to be formed relatively thick. The pits are buried by the paint. The substrate may be swollen by the solvent. In addition, it is difficult to uniformly coat the resin paint. According to the method which utilizes glow discharge, parallel electrodes are disposed in an atmosphere containing a monomer with a relatively small distance therebetween. The substrate is placed on one electrode. A voltage is applied across both electrodes to cause glow discharge to thereby generate a plasma. The monomer is thus polymerized to form a polymer layer on the substrate. According to the method utilizing glow discharge, since the field of activation of the monomer (field of plasma generation) is the same as the field of polymerization, the polymer layer formed on the surface of the substrate is constantly exposed to the discharge. As a result, fine pin holes tend to form. Furthermore, a polymer layer of uniform thickness may not be obtained unless the distance between the parallel electrodes is constant. The reaction is strong and is hard to control.
When a polymer is sputtered, it evaporates in the form of a monomer or an oligomer such as a dimer, and polymerizes on the substrate or before it reaches the substrate. For this reason, the polymer is deposited on the surface of the substrate in the form of a rough network, spots, islands or masses. In order to form a uniform layer, the layer must be made considerably thick. Such a thick layer is not suitable as a dielectric layer for a video disc.